After learning so much about planted tanks and cycling a tank I just realized something. I have always treated the water going into the tank but what about the water that is already in there? does it need to be treated again?

QUESTION:
During a water change I add 2 gallons of water to a 4 gallon tank do I treat only the 2 gallons or all 4 gallons? I use Seachem Prime btw.

When you add 2 gallons of new water to the tank, only the new water needs to be treated.

Example: You have a 4 gal tank.
(a) You do a 100% water change, adding 4 gal of new water. All 4 gallons of new water must be treated with the appropriate amount of conditioner.
(b) You do a 50% water change, adding 2 gal of new water. Only the 2 gal of new water need to be treated with the appropriate amount of conditioner.

Conditioners neutralize/remove chlorine, chloramines and heavy metals. The process of removing/neutralizing these is permanent. So the conditioner doesn't need to be added again.

Prime also temporarily neutralizes ammonia. So if you're using it with that specific purpose in mind, then yes, you may need to redose. (You can do a forum search to find more info, if you're using Prime for this purpose. I don't use it, so I don't recall the exact parameters involved.)